It seems random, but I know nothing with computers is really random. I had no clues as to what was happening, until I was logged in with ssh from another computer when it froze. I had done "ls -la," then "man find" (to hunt for core dump files) when it hung over ssh, then I found this on the screen of the problematic computer and managed to get a photo:

I don't know what came before that because I couldn't scroll up. It said it would reboot in 15 seconds, but it didn't. Here is dmesg for the next reboot:

* Motherboard is an ASUS M3N78-VM
* Processor is an AMD AthlonII X2-250
* Boot HD (ad4) is a Seagate SATA 1tb
* 2nd HD (ad1) is a Seagate IDE 320gb with an old FreeBSD installation not automatically mounted and not listed in fstab. (It was not mounted when the freeze happened, it is only there for me to get previous data from)

ACPI 2.0 support is disabled in bios (default setting). I tried disabling ACPI APIC support in the bios, but it gave me the "mountroot>" prompt when I tried booting. Choosing the Option 2 from the FBSD boot screen (Boot without ACPI) also gave the "mountroot>" prompt. It boots normally otherwise.

Does anyone have any suggestions for me to fix the freezing? I gather from searches that it may have something to do with the computer losing contact with the HD, but have not found a solution (or an explanation).